1. Field of the Invention
The present invention relates to analytical techniques for determining the composition, phase, structure, or other properties of a sample of material.
2. Background of the Invention
Thermal analysis techniques generally comprise measuring a physical parameter as a function of the temperature of the sample. The sample temperature is strictly controlled throughout the analysis. Whenever the sample undergoes a chemical or physical transformation, phase change or other transition which affects the physical parameter being measured, the changes in that physical parameter may be interpreted to analyze the composition, structure, or thermal stability of the sample.
One common thermal analysis technique is thermogravimetric analysis ("TGA"). TGA is a thermal analysis technique which measures the weight change of a material as a function of temperature, or as a function of time at a controlled temperature. The classic TGA method comprises heating the sample at a constant rate of temperature increase, typically at 10.degree. C. to 50.degree. C. per minute, while the sample weight change or the percent of weight change is recorded versus temperature.
Other thermal analysis techniques include Differential Thermal Analysis (DTA), Differential Scanning Calorimetry (DSC), Pressure Differential Scanning Calorimetry (PDSC), Thermomechanical Analysis (TMA), Dynamic Mechanical Analysis (DMA), Dynamic Mechanical Spectrometry (DMS), Dielectric Analysis (DEA), Differential Photocalorimetry (DPC), Thermal Conductivity Analysis (TCA), and any simultaneous combination of these techniques.
Differential Scanning Calorimetry measures the temperatures and the heat flow associated with transitions in materials as a function of time and temperature. These measurements provide quantitative and qualitative information about the sample transitions that involve endothermic or exothermic processes, or changes in heat capacity. Pressure Differential Scanning Calorimetry is a related technique in which the heat flow and temperature of transitions are measured as a function of temperature under controlled pressure.
Differential Thermal Analysis, like DSC, measures the temperatures and heat flow associated with transitions in materials as a function of time and temperature. However, unlike DSC, DTA results are semi-quantitative. DTA is generally carried out at higher temperatures than DSC.
Thermomechanical Analysis measures linear or volumetric changes in materials as a function of temperature under controlled stress or strain.
Dynamic Mechanical Analysis and Dynamic Mechanical Spectrometry measure mechanical properties of a material as it is deformed under periodic stress as a function of temperature.
Dielectric Analysis measures the dielectric properties of materials as a function of temperature.
Differential Photocalorimetry measures the heat absorbed or released by a sample as it and an inert reference are exposed simultaneously to radiation of known wavelength and intensity.
Thermal Conductivity Analysis measures the thermal conductivity of materials as a function of temperature.
Conventional thermal analysis techniques have limited resolution because in conventional thermal analysis, time and temperature are changing simultaneously. Because chemical and physical transformations are time-dependent (not instantaneous), transitions that actually occur as a function of time are recorded as occurring as a function of temperature.
TGA is particularly useful for observing the thermal decomposition of compounds. When individual thermal decompositions occur at well separated temperatures, quantitative information about sample composition may be obtained from the percent weight change per minute at each transition. However, due to the limited resolution of conventional TGA, in conventional TGA decomposition transitions frequently overlap or appear drawn out in temperature. This substantially reduces the ability to obtain an accurate measurement of weight change and reaction temperature.
It has long been known that the use of very slow TGA heating rates will improve the separation of some overlapping transitions and, thus, increases the resolution of the technique. U.S. Pat. No. 3,344,654 to Erdey, et al. ("Erdey"), which is incorporated by reference herein, discloses a quasi-static technique of reducing heating rate to limit the rate of weight change to a predetermined maximum during transitions. Although using very slow heating rates or heating rates controlled by sample weight change improves the separation of transitions, such methods also increase substantially the total time required for a measurement, thereby reducing laboratory productivity.
Moreover, increasing measurement time reduces the accuracy and reliability of the analysis. For example, a sample exposed to high temperatures for a long period of time undergoes slow time-dependent changes such as oxidation, deformation, absorption and adsorption, which may introduce errors in the analysis. The long time period also implies further difficulties due to instrument drift, and to ambient temperature, humidity, and pressure variations. Mechanical vibrations and fluctuations in the voltage of the main power line also increasingly affect the accuracy of the analysis as the analysis time increases. Furthermore, the effective signal-to-noise ratio is reduced, because the signal peaks are flattened by the slow temperature ramp.
Quasi-static techniques are discussed extensively in the article by F. Paulik and J. Paulik, "Thermoanalytical Examination Under Quasi-Isothermal--Quasi-Isobaric Conditions." Thermochimica Acta, Vol. 100 (1986), pp. 23-59, which is incorporated by reference herein. The Paulik quasi-isothermal technique attempts to maintain a specific rate of weight change by controlling the temperature of the sample.